[x86] Teach the shuffle mask equivalence test to look through build

[x86] Teach the shuffle mask equivalence test to look through build
vectors and detect equivalent inputs.

This lets the code match unpck-style instructions when only one of the
inputs are lined up but the other input is a splat and so which lanes we
pull from doesn't matter. Today, this doesn't really happen, but just by
accident. I have a patch that normalizes how we shuffle splats, and with
that patch this will be necessary for a lot of the mask equivalence
tests to work.

I don't really know how to write a test case for this specific change
until the other change lands though.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@229307 91177308-0d34-0410-b5e6-96231b3b80d8